cambium

a layer of delicate meristematic tissue between the inner bark or phloem and the wood or xylem, which produces new phloem on the outside and new xylem on the inside in stems, roots, etc., originating all secondary growth in plants and forming the annual rings of wood.

In plants, a layer of actively dividing cells between xylem (fluid-conducting) and phloem (food-conducting) tissues that is responsible for the secondary growth of stems and roots, resulting in an increase in thickness.

in plants, layer of actively dividing cells between xylem (wood) and phloem (bast) tissues that is responsible for the secondary growth of stems and roots (secondary growth occurs after the first season and results in increase in thickness). Theoretically, the cambium is a single layer of cells, called initial cells; practically, it is difficult to distinguish the initials from their still-undifferentiated daughter cells, and several cell layers are collectively called the cambium, or cambial zone. Cambial cells divide to produce secondary xylem cells toward the central axis of the stem and secondary phloem cells toward the outside. The cambium originates from undifferentiated cells that have retained their embryonic capacity for continued growth and differentiation. A cambium may also form within callus tissues—masses of cells that grow over the injured surface of a wound, leading to healing.

Cambium — Cam bi*um, n. [LL. cambium exchange, fr. L. cambire to exchange. It was supposed that cambium was sap changing into wood.] 1. (Bot.) A series of formative cells lying outside of the wood proper and inside of the inner bark. The growth of new wood … The Collaborative International Dictionary of English